This invention relates to magnetic field sensors.
A schematic design of a simple type of conventional fluxgate magnetometer is shown in the accompanying FIG. 1. The magnetometer consists of a high permeability ferromagnetic core 1 with a primary winding or excitation coil 2 and a secondary or detection winding 3. The primary winding 2 is excited by a sine wave or triangular wave current derived from a signal generator 4. A band pass filter 5 may be necessary to remove the second harmonic component from the primary winding exciting current.
The amplitude of the primary winding exciting current is adjusted in order to drive the core 1 into saturation. Graphs of the magnetic field intensity (H) in the core, which is proportional to the primary winding exciting current, and the flux density (B) in the core versus time are shown in the accompanying FIGS. 2a and 2b, respectively (solid lines). These graphs assume a triangular waveform from the signal generator 4 and that the B-H curve for the core material is linear up to saturation and then becomes flat (FIG. 2c).
The voltage output from the secondary winding 3 is proportional to dB/dt and hence is a rectangular wave as shown in FIG. 2d. If the B versus time waveform (FIG. 2b) is symmetric then the positive and negative pulses of the secondary winding voltage are equal in amplitude and 180.degree. out of phase. Analysis of this waveform has shown that the second harmonic component is zero. If this conventional fluxgate magnetometer is placed in a d.c. magnetic field which is directed along the core 1, as indicated by the arrows in FIG. 1, then an offset is produced in the H versus time and B versus time graphs, as indicated by the dashed lines in FIGS. 2a and 2b. This leads to a phase shift in the secondary winding voltage, such that the positive and negative pulses are no longer 180.degree. out of phase, but rather are as indicated by the dashed lines in FIG. 2d. The second harmonic components of the positive and negative signals no longer cancel, and a resultant second harmonic signal is produced which for small fields is proportional to the field. The secondary winding output voltage may be bandpass filtered as at 6 (FIG. 1) in order to remove signal components other than the second harmonic component, which is amlified as at 7 and then detected by a phase sensitive detector 8. The polarity of the output as indicated on meter 9 is determined by the direction of the d.c. magnetic field. A frequency doubler 10 serves to provide a reference for detector 8.
Another type of magnetic field sensor is the Hall effect device which, as is well known, comprises a platelike body of a semiconductor material through which a transverse magnetic field may be applied. The effect of the magnetic field is to deflect an electric current flowing across the body between a pair of current electrodes, this deflection of the current introducing a potential difference (Hall voltage) between a pair of sensor or Hall electrodes disposed one each side of a line joining the two current electrodes. In order that Hall effect devices may have the ability to sense weak magnetic fields (10-100 gamma and below), it has previously been proposed to provide the devices with flux concentrators as, for example, disclosed in our British Application No. 8111812 (Ser. No. 2081973) (G. D. Pitt et al) or our British Application No. 8318267 (G. D. Pitt-P. Extance). The latter specification describes the use of a thin ribbon of a magnetic material of high permeability, for example the metallic glass ribbon sold under the trade name "Vitrovac 6025", for the flux concentrators. The plate-like body of semiconductor material may, for example, comprise GaAs. Hall effect devices with metallic glass flux concentrators have sensitivities over two orders of magnitude higher than the conventional Hall effect sensors. However, the offset voltage at zero field and the noise/drift levels limit their usefulness.